The present invention relates to a transmission control system for an automatic transmission for an automotive vehicle such as a passenger car, and more particularly relates to a transmission control system for an automatic transmission for an automotive vehicle such as a passenger car or the like, in which the actual decision as to what speed stage of the transmission should be chosen to be engaged at each particular time is performed electrically by an electrical transmission control device in response to electrical signals indicative of the current operational conditions of the vehicle (such as engine load and vehicle road speed), and in which the selective supplying of hydraulic fluid pressure to at least one hydraulic fluid pressure actuated friction engaging mechanism of the transmission, in order to engage said chosen transmission speed stage, is performed by an electrically actuated hydraulic fluid pressure switching valve.
Generally, an automatic transmission for an automotive vehicle comprises a gear transmission mechanism and a transmission control system. The gear transmission mechanism is constructed so as to be able to provide a plurality of different rotational speed ratios between its power input shaft and its power output shaft, according to the pattern of selective supply of actuating hydraulic fluid pressure to one or more hydraulic fluid pressure actuated friction engaging mechanisms comprised within said gear transmission mechanism such as hydraulically actuated clutches and hydraulically actuated brakes; and the transmission control system is constructed so as to decide, based upon the current operational parameters of the vehicle, conventionally including the current values of a parameter representative of engine load and of a parameter representative of vehicle road speed, what speed stage is the proper speed stage for said gear transmission mechanism to be currently providing, i.e. what speed stage is the most appropriate for the vehicle incorporating this automatic transmission to be operated in at the current time. Further, said transmission control system is constructed so as to implement this speed stage decision which it makes by providing appropriate patterns of hydraulic fluid pressure supply and non supply to the aforesaid hydraulic fluid pressure actuated friction engaging mechanisms of the gear transmission mechanism such as the hydraulic clutches and hydraulic brakes.
Thus, when it is decided by the transmission control system that it is an appropriate time for a change of the selected speed stage of the gear transmission mechanism from the currently selected speed stage to a new speed stage which is now newly desired, then said transmission control system alters the pattern of selective supply and non supply of hydraulic fluid pressure to the hydraulic fluid pressure actuated friction engaging mechanisms of the gear transmission mechanism, thus altering the pattern of engagement and non engagement of said hydraulic fluid pressure actuated friction engaging mechanisms, and thereby the gear transmission mechanism is shifted from its earlier selected speed stage to its currently and newly desired speed stage.
Conventionally in the prior art transmission control systems have been hydraulic ones. In such a hydraulic transmission control system, a throttle hydraulic fluid pressure control valve is provided, which senses the amount of opening of a throttle valve of the internal combustion engine of the vehicle (this throttle opening amount being taken as a parameter indicative of vehicle engine load) and which modulates a source of hydraulic fluid pressure, so as to output said modulated hydraulic fluid pressure as a hydraulic fluid pressure signal (hereinafter to be called the throttle hydraulic fluid pressure signal), substantially proportional to said throttle opening, i.e. substantially proportional to vehicle engine load; and also a governor hydraulic fluid pressure control valve is provided, which senses the revolution speed of the power output shaft of the gear transmission mechanism or of some other member thereof (this revolution speed being taken as a parameter indicative of vehicle road speed) and which modulates a source of hydraulic fluid pressure, so as to output said modulated hydraulic fluid pressure as a hydraulic fluid pressure signal (hereinafter to be called the governor hydraulic fluid pressure signal), substantially proportional to the revolution speed of said power output shaft or other member, i.e. substantially proportional to vehicle road speed.
In such a prior art type of hydraulic transmission control system, these two hydraulic fluid pressure signals, i.e. the throttle hydraulic fluid pressure signal and the governor hydraulic fluid pressure signal, are supplied to a hydraulic speed stage decision making system, which includes one or a plurality of hydraulic speed shifting valves. In each of these one or more hydraulic speed shifting valves, a spool element or valve element is provided: and the throttle hydraulic fluid pressure and the governor hydraulic fluid pressure act in opposition on each of these valve elements of each of these hydraulic speed shifting valves, each of these valve elements being driven by the throttle hydraulic fluid pressure and the governor hydraulic fluid pressure in opposite directions. Further, typically other biasing forces such as spring forces and other hydraulic forces also act on each of these valve elements: and thus in each of these hydraulic speed shifting valves the spool or valve element thereof moves to and fro according to a predetermined balance relationship between the throttle hydraulic fluid pressure and the governor hydraulic fluid pressure, being moved in a one direction when the throttle hydraulic fluid pressure overcomes the governor hydraulic fluid pressure in its biasing action on the valve element of said hydraulic speed shifting valve, and being moved in the direction opposite to said one direction when the governor hydraulic fluid pressure overcomes the throttle hydraulic fluid pressure in its biasing action on said valve element of said hydraulic speed shifting valve.
Each of these hydraulic speed shifting valves controls, according to the position to which its valve element is moved, selective supply of actuating hydraulic fluid pressure from a source of hydraulic fluid pressure to one or more of the aforesaid hydraulic fluid pressure actuated friction engaging mechanisms such as hydraulic clutches and/or hydraulic brakes; and thus the different combinations of engagement and non engagement of said hydraulic fluid pressure actuated friction engaging mechanisms required for providing each of the speed stages of the gear transmission mechanism are provided by the transmission control system, according to the current operational state of the vehicle as indicated by said throttle hydraulic fluid pressure, by said governor hydraulic fluid pressure, and possibly by other operational parameters of the vehicle, which may be similarly converted into hydraulic fluid pressures for biasing the aforesaid valve elements of the hydraulic speed shifting valves.
On the other hand, it has been recently widely developed to operate the speed shifting valves by electromagnetic actuators operated by an electronic computer control means which determines the speed shifting points by electronic computation based upon input data with regard to the throttle opening, the vehicle road speed, etc. In these electronic transmission control systems, the amount of throttle opening is given as the throttle hydraulic fluid pressure and the amount of vehicle road speed is given as the governor hydraulic fluid pressure.
Typically in such prior art transmission control systems the supply of hydraulic fluid pressure for being switched as explained above by the speed shifting valves and for thus being selectively supplied to the hydraulic fluid pressure actuated friction engaging mechanisms of the gear transmission mechanism has been a line hydraulic fluid pressure, which has also been used as a source of pressure for being modulated by the aforesaid throttle hydraulic fluid pressure control valve and the aforesaid governor hydraulic fluid pressure control valve, so as to produce respectively said throttle hydraulic fluid pressure and said governor hydraulic fluid pressure. This has given rise to some imperfections in the such a prior art hydraulic system of transmission control, which will now be explained.
When a hydraulic fluid pressure actuated friction engaging mechanism such as a hydraulic clutch or a hydraulic brake is engaged from the disengaged condition by being supplied with actuating hydraulic fluid pressure from the state of not being supplied with actuating hydraulic fluid pressure, then it rotationally couples together two elements (which in future in this specification will be referred to as the coupled elements) which formerly were rotating with respect to one another. For example, these coupled elements may typically, in the case of a hydraulic fluid pressure actuated clutch, be two sets of clutch plate elements which are squeezed together by an element such as a piston element which is driven by said actuating hydraulic fluid pressure, or may be, in the case of a hydraulic fluid pressure actuated brake, two sets of brake plate elements which are again squeezed together by an element such as a piston element which is driven by said actuating hydraulic fluid pressure. When the element such as a piston element which is driven by said actuating hydraulic fluid pressure first presses together said coupled elements, which are rotating with respect to one another, then when these coupled elements first meet one another naturally they slide against one another, but since they are being pressed together by the biasing force of the actuating hydraulic fluid pressure which is acting via said element such as a piston element, soon these coupled elements grip against one another and cease to rotate with respect to one another. Thus the hydraulic fluid pressure actuated clutch or the hydraulic fluid pressure actuated brake engages, i.e. performes the action of so called "biting".
For the shifting of the gear transmission mechanism which includes this hydraulic fluid pressure actuated friction engaging mechanism to be performed in the most desirable manner, i.e. in the smoothest manner without too much slipping, it is desirable that this biting of the friction engaging mechanism should not occur either too quickly or too slowly. In more detail, if the coupled elements of the hydraulic fluid pressure actuated friction engaging mechanism reach rotational rest with respect to one another too quickly, i.e. if the hydraulic fluid pressure actuated friction engaging mechanism bites too quickly, which will occur in the event that the actuating hydraulic fluid pressure therefor is too high, then the shifting of the gear transmission mechanism will be rather jerky and substantial shift shock will occur during such a change of transmission speed stage, which can seriously deteriorate the service life of the automatic transmission as a whole, as well as possibly damaging other components of the vehicle drive train. On the other hand, if the coupled elements of the hydraulic fluid pressure actuated friction engaging mechanism reach rotational rest with respect to one another too slowly, i.e. if the hydraulic fluid pressure actuated friction engaging mechanism bites too slowly, which will occur in the event that the actuating hydraulic fluid pressure therefor is too low, then the shifting of the gear transmission mechanism will be too slow, and serious slippage during said shifting of the gear transmission mechanism may occur. In this case, over revving of the internal combustion engine of the vehicle incorporating the automatic transmission may well occur, which will aggravate the effect of said slippage. This slippage effect can seriously deteriorate the service life of the automatic transmission as a whole, possibly even causing sudden failure thereof, due to burning out of the coupled members of the hydraulic fluid pressure actuated friction engaging mechanism or the like by the generation of a large amount of heat due to friction.
In order to keep the speed of engagement of the hydraulic fluid pressure actuated friction engaging mechanisms neither too fast nor too slow, therefore, the actuating hydraulic fluid pressure for these hydraulic fluid pressure actuated friction engaging mechanisms should be regulated to an appropriate level, neither too high nor too low. This appropriate hydraulic fluid pressure level should be varied according to the amount of torque that the hydraulic fluid pressure actuated friction engaging mechanism is going to transmit, with further consideration of the condition that the hydraulic fluid pressure actuated friction engaging mechanism has been finally engaged at a certain speed stage to transmit a required torque, and it is known that the appropriate hydraulic fluid pressure level should actually be varied approximately proportionally to the amount of torque to be transmitted in the low to medium engine load range and should be kept substantially constant or should increase by a small amount as the torque to be transmitted increases in the middle to high engine load range. Such a variation in the actuating hydraulic fluid pressure for the hydraulic fluid pressure actuated friction engaging mechanisms of an automatic transmission system for a vehicle, by keeping the pressure required for the supply of said actuating hydraulic fluid pressure as low as possible, further has the desirable effect of minimizing the amount of energy produced by the internal combustion engine of the vehicle which is required for pumping hydraulic fluid in the transmission and in the transmission control system.
Conventionally, this desirable regulation according to the engine load of the line hydraulic fluid pressure which is used as a source for the actuating hydraulic fluid pressure which is selectively supplied to the hydraulic fluid pressure actuated friction engaging mechanisms of the gear transmission mechanism of the automatic transmission, has been done according to the throttle hydraulic fluid pressure. When the line hydraulic fluid pressure is thus modified in accordance with the throttle hydraulic fluid pressure, it becomes sometimes difficult to design the behavior of the throttle hydraulic fluid pressure to be optimum for the design of the speed shift points of the gear transmission mechanism and also to design the behavior of the line hydraulic fluid pressure to be optimum for obtaining the abovementioned appropriate hydraulic fluid pressure level characteristics for the engagement and disengagement of the hydraulic fluid pressure actuated friction engaging mechanisms.
Accordingly, therefore, since the preservation of the correct speed stage shift points for the gear transmission mechanism of the automatic transmission is of paramount importance, in view of the necessity to provide proper transmission operation, the optimization from the point of view of good engagement of the hydraulic fluid pressure actuated friction engaging mechanisms of the behavior of variation of the line hydraulic fluid pressure in relation to variation of the load on the internal combustion engine of the vehicle has been required to be sacrificed, in the design of prior art automatic gear transmission mechanism control systems.